Recently, there has been an increasing interest in energy storage technology. As the application fields of energy storage technologies have been extended to cellular phones, camcorders, notebook computers, PCs and electric cars, efforts have been increasingly been made towards the research and development of electrochemical devices. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, extensive research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy.
Many secondary batteries are currently available. Among these, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni—Cd, and H2SO4—Pb batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly prohibit the dangers (such as fire and smoke emission) of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or a puncture of a separator may pose an increased risk of explosion. In particular, when a porous substrate of a non-woven fabric is used as a separator for electrochemical devices, costs for preparing the separator can be reduced, but the mechanical strength thereof becomes weak due to the low tensile strength, tensile modulus and flexural modulus of the porous substrate and the insufficient binding of fiber strands in the porous substrate. For this reason, the separator using the non-woven porous substrate may be ruptured during the preparation of electrochemical devices. Also, due to the non-woven fabric substrate having large pores, a leak current may occur during battery operation to deteriorate the insulating property of the separator.